1. Field of the Invention
The present invention generally relates to remote condition sensors and measurement devices and, more particularly, to remote measurement and sensing using optical fiber communication links.
2. Description of the Prior Art
Condition sensing and measurement are encountered in many diverse fields of endeavor. Many of these applications require very precise measurement of a physical parameter and often at extremely inaccessible locations and/or where severe conditions exist. For example, so-called down-hole telemetry in petroleum exploration or production, the sensor may be required to communicate with the remainder of the measurement system over substantial distances while making extremely accurate and highly sensitive measurements of strain, pressure or temperature in an environment of very high temperatures and pressures which may adversely affect calibration of the sensor, compromise accuracy or limit resolution of the measurement through environmental effects on the communication link including electrical or magnetic noise.
So-called optical fiber sensors employing a transducer exploiting an optical effect and communicating over a length of optical fiber are known and used in some of the more adverse measurement environments. Optical fibers are not susceptible to the pick-up of electrical or magnetic noise and optical energy transmitted thereto can often provide a reference measurement for calibration or correction of a measured value. For example, transmission and return of optical signals over the same or a pair of optical fibers which are constrained to have the same geometrical configuration (e.g. by identity or being joined together) can provide compensation for optical signal losses in the fiber. The use of optical fibers also allows compensation to be determined through use of spectrally separated optical signals.
However, transmission of light through optical fibers, while efficient, is imperfect and attenuation of the light signal energy will occur over the length of any optical fiber. Spectral separation of optical signals is generally accomplished with passive optical filters to avoid introduction of additional variables and potential non-linearities into the measurement with active devices. Passive optical filters are also less than fully transmissive to any wavelength and greatly reduce the energy of a broad band light source by their filtering action which strongly attenuates a range of wavelengths and for which the filter is employed. When filters are employed in the transducer (as opposed to the detector end of the system) both of these factors limit the accuracy of compensation of optical fiber sensors and limit the length of optical fiber over which communication with the transducer may be accomplished within practical sensitivities of energy sensors and noise immunity of the optical portion of the measurement system.
Returning to the down-hole telemetry example alluded to above, the capabilities of drilling equipment at the present state of the art permits wells substantially in excess of one mile in depth and presenting increased levels of temperature and pressure where measurements of extremely high accuracy and resolution are desirable. However, known passive optical fiber sensors are not capable of producing accurate, high resolution and/or compensated absolute measurements under such conditions and over such distances.
Further, it is not practical to even approximate or simulate the conditions of temperature and pressure which will be encountered in such an environment. Therefore, calibration of the transducer and communication link for such conditions is similarly not practical and any attempt to do so, particularly over the distances and variety of environmental conditions which may be encountered by the fiber optic link would compromise the measurement sought to be made. Further, known compensated optical fiber sensors are not capable of reliable operation, if operable at all, with ultra-high resolution and accuracy over distances for which telemetry is currently required.
Additionally, extremes of temperature and, possibly, pressure can permanently or reversibly alter the optical properties of filters if used in passive optical transducers (e.g. by subtle alteration of pigment composition, crystal lattice spacing or polarizing effects due to strain), as is generally the case in known transducers. While systems are known in which such changes of optical properties may not be important, the possibility of changes in optical properties may be a source of error or variability in measurement systems